Resistor



Dec. 25, 1934. A. L. PUGH. JR

masrswon Filed Nov. 8, 1930 Til/75L A TION DEV/CE- INVENT OR ATTORNEY:

. Patented Dec. 25, 1934 UNITED STATES PATENT OFFICE RESISTORApplication November 8, 1930, Serial No. 494,281

8 Claims.

This invention relates to resistance devices, particularly to wire-woundresistance devices.

An object of the invention is to provide a resistance device of highprecision. Another object of the invention is to provide a resistancedevice of relatively low ohmic resistance and high precision. Anotherobject of the invention is to provide a resistance device in which theeffects of thermal electromotive forces are minimized so as to benegligible or are neutralized. Another object of the invention is toprovide a resistance device having a low temperature coefficient ofresistance. Another object of the invention is to provide a resistancedevice having a relatively low temperature coefllcient and adapted togenerate a counter thermal electro-motive force to neutralize anundesired electro-motive force. Another object of the invention consistsin providing a resistance device which, when connected to the terminalsof a particular type of circuit, reduces the thermal electromotive forceset up in the circuit to a negligible quantity. Another object of theinvention is to provide a sectionalized resistance device in whichcurrent variations or microphonic characteristics at the junction of thewindings are largely eliminated in order, for example, to make thedevice noiseless in an audio circuit.

Another object of the invention is to provide a method of makingresistance devices, having one or more of the above characteristics,which may be readily carried out in practice. Another object of theinvention isto provide a method of making resistance devices, having oneor more of the above characteristics, whereby the articles may beproduced at minimum cost. Another object of the invention is to providea method of producing resistance devices of relatively low resistanceand high precision, which shall be mechanically and electrically sound.Other objects will be in part obvious or in part pointed outhereinafter.

The invention accordingly consists in the features of construction,combinations of elements,

' arrangements of parts, and in the several steps and relation and orderof each of said steps to one or more of the others thereof, all as willbe illustratively described herein, and the scope of the application ofwhich will be indicated in the following claims.

In the accompanying drawing, in which is shown one of various possibleembodiments of the mechanical features of this invention,

Figure 1 is an axial sectional view of the resistance device,

Figure 2 is a view'showing the resistance device and connectingterminals in elevation, and a circuit in which the resistance device isconnected in diagram,

Figure 3 is an elevation of the form and windings, before the terminalsare integrally connected, and

Figure 4 is an elevation of a binding post terminal.

Similar reference characters refer to similar parts throughout theseveral views of the drawmg.

The resistance wire of the resistance device is preferably wound upon aceramic form 10, Fig ures 1 and 3, which has grooves 11 and 12 for theresistance wire, separated by central flanges 13, 13 integrally formedon the form 10, and bounded by end flanges 14, 14 which are alsointegrally formed on the form of ceramic material. By this construction,two entirely separate grooves for resistance wire are provided, thesegrooves being bounded by insulating flanges, and the wire being adaptedto be wound upon cylindrical portions of insulating material, preferablyceramic material.

I first coat the cylindrical portions and the flanges of the grooves 11and 12 with bakelite varnish, in order to make the winding easier,

and in order to prevent the ceramic material, which, in its naturalstate, is somewhat of an abrasive, from injuring the insulation of thewire being wound during the winding process.

Extending longitudinally through the form 10 is a central axial bore 15,and thus the form may be mounted upon an arbor for rotation of saidform. At both ends of the form 10 are cylindrical portions 16, 16, oneof which may be gripped by a chuck or the like of any suitable windingapparatus adapted to rotate the form 10, in order that resistance wiremay be wound into one of the grooves, for example the groove 11.Preferably by means of such a winding apparatus, I wind a desired amountof relatively high resistance wire 17 into the groove 11. Preferablythis wire has high resistance, and also a low temperature coefficient.Preferably the greater amount (for example from 93% to 99%) of the totalresistance of the device represented by the winding 1'7.

In order to wind the high resistance wire 17 into the groove 11, Ipreferably secure one end thereof to a chuck and cause the form 10 to berotated, thus winding the wire into the groove 11. In the commercialproduction of resistance devices of this general character, it isdesirable to have a standard form upon which to'wind the wire. It ishighly desirable, therefore, to have definite places where the wireemerges from the winding and leads to binding post terminals, or otherdevices, whereby the entire unit is placed in circuit with otherelectrical apparatus. It rarely happens, however, that the exactresistance length of wire desired has a lineal length exactly fittingthe form and adapted to fit in the place provided for it without leavingthe wire loose. In the attempt to make a device of the right resistancelength, and at the same time free from objectionable loose coils, muchtime has heretofore been wasted. I therefore wind as much of theresistance wire 17, which is preferably nickel alloy wire, into thegroove 11 as will make a total resistance slightly less than theresistance desired for the entire device, so that the resistance may becompleted to within precision limits by a relatively very small numberof turns of low resistance wire. Stating the matter in another way, itis diflicult, if not impossible, in the commercial production ofresistance devices of this character to bring the total resistance tothe desired amount with high resistance wire, because as the desiredresistance is approached, one more turn may make the resistance toohigh, or one less turn may make the resistance too low. I thereforecarefully check the resistance of the nickel alloy wire being wound andstop winding it and cut the end thereof when the resistance length isjust below the entire resistance desired. This leaves the form withturns of wire 17 in the groove 11 and two loose ends of wire projectingfrom it. As an example of what I mean by a nickel alloy wire, a coppernickel wire of 60% copper and 40% nickel may be used.

Some of the advantages of the invention would be secured by completingthe resistance device with a winding, in the groove 12, of nickel alloywire of large diameter, and therefore of low resistance. The desiredresistance value of the entire device could thus be reached within highprecision limits. But to attain other objects of the invention, I preferto wind wire 18 into the groove 12 of a metal other than nickel alloy,and preferably I use copper wire. I use copper wire in order to preventthe generation of an undesired net thermal electro-motive force whichwould otherwise (that is, with the use of a winding 18 of nickel alloywire) be generated, for the following reasons:

Resistance devices of this general type are adapted to be used-in manykinds of electrical circuits, butv in most electrical apparatus wherethey might be used connecting terminals or electrodes are made ofcopper. Copper, of course, is widely used because of its highconductivity, its relative cheapness when used in small quantities, ahdits malleability and ductility, which permit it to be forced into anyshape desired. As all metals have a definite place in the thermalelectric series, and as nickel alloy wire is separated by a substantialdistance from copper in this series. and as in fact most metals of lowconductivity are spaced apart in this series from copper, whatever heatthe entire resistance device generates produces, by reason of the thermocouple including the end 17a of the winding 1'7 and the outside copperterminal, a thermal electro-motive force of some magnitude. Assuming thewinding 18 to be of nickel alloy wire, a thermo couple opposite inpotential to the aforesaid thermo couple would exist at the opposite endof the device, including motive force of substantially the samemagnitude and opposite in direction. I therefore wind the groove 12 witha winding 18, preferably of copper wire, and as the connecting ends 17band 18b of the windings 17 and 18 are located adjacent the winding 17,the thermo couple constituted thereby is subject to as much heating asthe thermo couple formed by the end 17a of the winding 17 and theoutside copper terminal, and consequently the first thermalelectro-motive force is balanced by an equal thermal electro-motiveforce acting in the opposite direction, and the entire device willgenerate practically no net thermal electro-motive force.

The winding of the wire 17 having been stopped, preferably at thatcomplete turn which leaves the resistance of the wound wire just belowthe desired resistance, I wind just enough copper wire 18 into thegroove 12 to bring the total resistance of the entire device to thedesired amount within very close limits. Owing to the fact that the wire18 is of relatively low resistance, the difference of resistance in theentire device produced by any single turn thereof is small as comparedwith the total resistance, and hence it is possible to obtain results ofhigh precision. The exact number of turns of wire 18 may be readilycalculated from its known resistance and the known resistance of theentire winding 17.

I now remove the insulation from the four ends of the two windings l7and 18 and pass the ends 171) and 182) through slots 20, 20 into acentral groove 21 which is bounded by the walls 13, 13 of the ceramicform 10. As is clearly shown in Figure 3, the end walls 14, 14 of theceramic form 10 have transverse holes 22, 22 therethrough, penetratingto the interior axial bore 15. I now pass the outer ends 17a and 18a ofthe windings 17 and 18 through the opposite holes 22, 22 respectively,and out of the opposite ends oi the bore 15, leaving the uninsulatedends of the wire projecting a slight distance from each end of thecylindrical end portions 16, 16. The device is now in the conditionshown in Figure 3. Desirably I twist together the ends 1% and 18b.

Referring now to Figure 4, the device preferably includes metal bindingposts 23, illustrated in that figure. These binding posts include ashank portion 24, a threaded portion 25, and a central enlarged portion26. I now insert the shanks 24 of the binding posts 23 into the oppositeends of the central axial bore 15, the shank 24 passing alongside theends of the two separate windings. Enough wire is provided to projectfrom the ends of the bore by such a distance that, when the bindingposts 23 are in position, the scraped ends, or the ends from which theinsulation has been removed, will project slightly beyond the enlargedportions 26.

I now relate the entire device to a suitable mold, and mold metal uponboth cylindrical end portions 16, 16 of the ceramic form 10, and in thecentral groove 21. The metal employed is desirably type metal which maybe readily molded, but

,of the binding posts 23.

. equal extent.

couples that the addition of a third metal at the any other metal whichforms a good electrical union and which has other desirablecharacteristics, such as the ability to adhere tightly to the ceramicform l0, may be employed. I thus form integral metal' end portions 27a,27b integrally embedding the binding posts 23 and firmly connecting themto the ceramic form 10, and also integrally embedding the ends 17a and18a of the windings 17 and 18 respectively in these metal end portions.The metal flows into the bore 15, but is quickly cooled by the wide areaof theceramic material which it contacts and also by the shank 24, andso does not go beyond the shank 24 union between the winding 17 and onemetal end piece 26 on the one hand, and the winding 18 and the othermetal end piece 26 on the other hand, is thus secured, and in fact themetal end pieces are integral with the windings. Microphonic effects orcurrent variations are entirely avoided.

The metal 28 in the central groove 21 serves to integrally unite andembed the ends 171) and 18b of the windings 17 and 18, thus entirelyeliminating microphonic effects or current variations at this junction.The metal 28 serves as the junction between the two windings 17 and 18,and its position relative to the winding 17 is the same as the positionof the cast metal end piece 27a. Consequently the rise in temperature,due to the heating of the winding 17, of the cast metal junction 28 issubstantially the same as the rise in temperature of the cast metal end'piece 27a. Preferably the mass of the cast metal portions 28 and 27a isabout the same. This rise in temperature is effective in generating, atthe thermo couple constituted by the ends 17b and 181), a thermalelectro-motive force of sufficient magnitude to balance the thermalelectro-motive force generated at the cast metal end piece 27a. Thewinding 17, which generates by far the greater amount of heat generatedby the entire device, is thus included between the two metal pieces 27aand 28 which are raised to substantially the same temperature. Thus theentire device is in perfect thermal electric balance despite theprovision of a winding of different metal on the same ceramic form 10.It may here be noted that the fact that the cast metal end piece 27a andthe binding post 23 are interposed between the end 17a of the winding 17and an outside terminal 29, either in series or in parallel, does notchange the magnitude of the thermal electro-motive force generated atthis end of the device to any substantial extent, because all theseparts are heated to an It is the general law of thermo same temperatureat a heated union between two metals does not change the net thermalelectromotive force.

I next coat the entire device with another coating of bakelite and bakethe same, so as to reinsulate any exposed portions from which theinsulation was removed during the manufacture of the device. As shown inFigure 2, I desirably provide nuts 31, 31 fitting the threads on thethreaded portions 25, 25 of the binding posts 23. In this way theresistance unit may be placed in circuit in any desired apparatus, beingshown in Figure 2 as securely clamped between copper terminals 29 and30.

The completed device is adapted to be used in any circuit where a knownamount of resistance is desired, and such a circuit is indicateddiagrammatically in Figure 2. In the circuit of Figure 2, which ismerely illustrative of any electrical cir- A perfect electrical cuit inwhich the device might be used, there is no net thermal electro-motiveforce generated although there are two thermo couples which generateequal and opposite electro-motive forces. In this circuit the winding 18generates, under normal conditions, a negligible amount of heat; henceit can have no eilect upon the temperature of the metal 28 serving asthe junction of the two windings 17 and 18. Atall events, the thermalelectric balance of the circuit cannot be disturbed because there is nothermo couple between the terminal 30 and the winding 18, as both aremade of copper. i

It will thus be seen that there has been provided by this invention amethod and apparatus in which the various objects hereinabove set forth,together with many thoroughly practical advantages, are successfullyachieved.

As various possible embodiments might be made of the mechanical featuresof the above invention, and as the art herein described might be variedin various parts, all without departing from the scope of the invention,it is to be understood that all matter herein set forth, or shown in theaccompanying drawing, is to be interpreted as illustrative and not in alimiting sense.

I claim:

1. A resistance device comprising a form having a pair of grooves, awinding of high resistance wire in one groove, a winding of lowresistance wire in the other groove, molded metal end pieces in whichthe opposite ends of the windings are integrally embedded, and a centralmolded metal piece in which the adjacent ends of the windings areintegrally embedded. v

2. A resistance device comprising a winding the end of which is embeddedin cast metal, a second winding having a different position in thethermal electric series from the first winding, and cast metal in whichthe ends of the two windings are integrally embedded.

3. A resistance device comprising a ceramic form, end walls provided bysaid ceramicform, a pair of walls provided in the central portion ofsaid ceramic form, resistance wire wound between one end wall and one ofthe central walls, resistance wire of low resistance wound between theother end wall and the other central wall, binding posts, integral castmetal end pieces holding said binding posts to said ceramic form, theends of said wire being embedded in said east end pieces and cast metalbetween the central walls in which the ends of the two windings areembedded.

4. A resistance device comprising a form, a relatively long winding ofhigh resistance wire upon said form, a relatively short precisionwinding of low resistance wire wound upon said form, a molded metalpiece embedding the adjacent ends of said windings, there being passagesfor passing the other ends of said windings through said form, saidpassages being existent in said form prior to the winding thereof andhaving predetermined circumferential locations, the entire unit having apredetermined resistance value within plus or minus a relatively smallresistance value.

5. A resistance device comprising a form, a

relatively long winding of high resistance wire wound upon said form, arelatively short precision winding of low resistance wire wound uponsaid form, a molded piece of metal in which the adjacent ends of saidwindings are embedded, and molded metal end pieces embedding theremaining ends of said windings, said remaining ends passing into saidmolded metal end pieces at predetermined places on said form, the entireunit having a predetermined resistance value within plus or minus arelatively small resistance value.

6. A resistance device comprising a form, a winding of relatively highresistance, a precision length of wire, the aforesaid winding beingcomposed of wire of a different metal, and a piece of relatively highspecific heat capacity connecting the precision length and. the windingto induce, when the piece is heated, a thermal electromotive forcecounter to the normal electromotive force generated at or adjacent theopposite ends of the resistance.

, 7. A resistance device comprising a form having a pair of grooves, anickel alloy winding in one groove, 2. copper winding in the othergroove, means connecting said windings forming a thermocouple, meansforming with the nickel alloy winding at the other end thereof and anoutside copper conductor an equal and opposite thermo couple, and heatabsorption means on either side of the nickel alloy winding positionedto receive substantially the same amount of heat from said windings.

8. A resistance device comprising a spindlelike form having a pair ofgrooves, a winding of resistance wire of high resistance wound in onegroove around the spindlelike form, a winding of resistance wire of lowresistance connected to the winding 01' high resistance and wound in theother groove around the spindlelike form, the two windings havingdifferent places in the thermal electric series, and molded metal inwhich .the ends of the wires are embedded.

ALEXANDER L. PUGH, JR.

